Aromatic hydrogenation to form cyclohexane with added nitrogen-containing compounds



United States Patent US. Cl. 260-667 9 Claims ABSTRACT OF THE DISCLOSURE Process for hydrogenating aromatics with less isomerization to S-membered carbon ring products. Hydrogenation is carried out in the presence of a nitrogen containing compound such as pyridine, ammonia and others. The hydrogenated product is substantially reduced in percentage of 5-membered ring components.

This invention relates to a process for hydrogenating aromatic hydrocarbons and more particularly is directed to a process for producing high purity cyclohexanes by reduction of aromatic compounds with hydrogen in the presence of a noble metal catalyst.

In the formation of saturated derivatives of aromatics, the aromatics are normally hydrogenated in the presence of a noble metal catalyst such as platinum, palladium, rhodium, etc., at a temperature of about 200 C. If the aromatic compounds contain a detrimental amount of sulfur, i.e. greater than p.p.m. or if the catalyst has become relatively inactive due to age, the temperature for hydrogenation must be increased above 200 C. Usually such high hydrogenation temperatures are avoided by pretreating the feedstock to remove the sulfur since isom erization occurs at higher temperatures and the formed 6-membered ring naphthenes or cyclohexanes are contaminated with 5-membered ring naphthenes or cyclopentanes. Additionally, the sulfur contained in the aromatic feedstock contaminates or poisons the noble metal catalyst increasing its isomerizing activity and decreasing its hydrogenating activity. Thus, it has been extremely difiicult to obtaincyclohexanes from feedstocks contaminated with sulfur without the accompanying formation of cyclopentanes unless the feedstock is first pretreated. Consequently, hydrogenation of sulfur-containing feedstocks in the past has generally required a twostep process in which desulfurization or hydrodesulfurization to below 10 ppm. sulfur has been effected using a hydrotreating catalyst such as cobalt molybdate, which is relatively insensitive to sulfur and other contaminants, prior to the hydrogenation step. The pretreatment usually also reduces the nitrogen to much less than 10 p.p.m.

The sulfur contamination may occur in the form of mercaptans, thiophenes, dissolved hydrogen sulfide, sulfones, thioethers, etc. Even normal low sulfur aromatic feedstocks contain up to 10 p.p.m. sulfur and the sulfur content in some feedstocks may go as high as 2500 p.p.m.

Commercially, cyclohexanes are produced primarily by either hydrogenation of aromatics such as benzene, toluene, and the like, or by distillation directly from petroleum. The hydrogenation process is carried out by contacting the aromatics and hydrogen in the presence of a hydrogenation catalyst. It was formerly thought necessary that the temperature be maintained from 150 to 250 C. and the pressure from 10 to several hundred atmospheres, in either a batch or continuous type operation.

It has now been found that higher temperatures can be tolerated in a one-stage process without detrimental 3,446,863 Patented May 27, 1969 amounts of isomerization even when the feedstock contains up to 2500 p.p.m. sulfur if certain nitrogen compounds are added to the feedstock. Substantially, complete hydrogenation can be obtained even in the presence of high sulfur at temperatures up to about 425 C. without harmful effects. It has further been found that low sulfur feedstocks are beneficially alfected by hydrogenating in the presence of certain nitrogen compounds.

It is an object of this invention to produce cyclohexanes from aromatics by hydrogenation without isomerization.

Another object of this invention is to provide a onestage process for producing cyclohexanes from aromatics containing sulfur contaminants.

Still another object of this invention is to provide a' process for hydrogenating aromatics at temperatures up to 425 C.

It is another object of this invention to provide a process for hydrogenating aromatic compounds to cyclohexanes without sulfur contamination of the catalyst and without concurrent formation of cyclopentanes.

Still another object of this invention is to provide a process for producing a product highly concentrated in cyclohexanes from an aromatic feedstock contaminated with sulfur by hydrogenation.

These and other objects of this invention will become more readily apparent in view of the appended claims and following description.

This invention comprises hydrogenating aromatics in the presence of a nitrogen containing compound and a noble metal catalyst. The nitrogen containing compound may be added to the hydrogen prior to hydrogenation, may be indigenous to the feedstock, or may be mixed with the feedstock prior to hydrogenation to obviate the detrimental effects of sulfur and to suppress the isomerization of cyclohexanes to cyclopentanes. Any nitrogenous compound which when hydrogenated forms ammonia, or, a stable amine at hydrogenation reactor conditions, may be added. Such compounds include those commonly found in petroleum and coal tar, these are pyridines and benzopyridines such as quinolines, isoquinolines, acridines and phenanthridines; pyrroles and benzopyrroles such as carbazoles and indoles; nitriles; amines (including aromatic amines, aliphatic amines and cyclicpolymethylenimines); and ammonia. Also included are the slower reacting nitrogen compounds which, nevertheless, form ammonia and function in the process of this invention. These compounds include nitroso and nitro compounds; amides; and amino acids. The nitrogen compound can be added or premixed with the hydrogen or the feedstock prior to hydrogenating the aromatic feed. Isomerization may also be suppressed by addition of the nitrogen containing compound to the hydrogenation reaction vessel prior to or during the hydrogenation.

The amount of nitrogen required to suppress isomerization to cyclopentanes and to reduce the isomerization induced by sulfur has been found to be at least 5 p.p.m. based on a sulfur-free feedstock and must be sufficient to give more than a 0.1 :1 atom ratio of nitrogen to sulfur in the reaction vessel. No detrimental effects have been observed for the addition of larger amounts of nitrogen, but the practical upper limit on nitrogen content is about 15,000 ppm. Favorable results have been obtained by adding as much as 13,400 p.p.m. ammonia (based on weight of aromatic feedstock) to the hydrogen for hydrogenating a feedstock containing a very small amount of sulfur. These results were obtained without a significant reduction in the hydrogenation level.

It has also been found that even where the initial aromatic feedstock contains substantially no sulfur, the conversion of aromatics to cyclohexane is made more selective by the addition of as low. as 5 p.p.m. of nitrogen containing compounds. This amount of nitrogen does not increase the temperature necessary for conversion of the aromatics to cyclohexane but does suppress the formation of cyclopentanes by isomerization of the cyclohexanes or adsorbed partially hydrogenated aromatics. While the effect of the nitrogen containing compound additives is more apparent when sulfur is contained in the feedstock, the nitrogen containing compounds are also effective in reducing high temperature isomerization of substantially sulfur-free feedstocks.

In preparation of cyclohexanes by the process of this invention, the temperature may be maintained between 280 and 425 C, the pressure may be maintained between 500 and 1500 p.s.i.g., the liquid hourly space velocity may be between .25 and 40, the atomic ratio of nitrogen to sulfur may be higher than .111, and the molar hydrogen-to-aromatic hydrocarbon ratio in the hydrogenation vessel may be 5:1 or greater with an upper limit at about 25:1 based on economics of the process. Any hydrogenation catalyst such as palladium or platinum can be used in the reaction chamber. For a further understanding of this reaction see my copending application Ser. No. 534,092 filed Mar. 14, 1966 and hereby incorporated by reference.

The preferred temperature for carrying out the process of this invention is from 200 to 400 C. since the reaction proceeds best at these temperatures with substantially complete hydrogenation and very little catalyst fouling. The preferred pressure is from 250 to 1500 p.s.i.g.

The amount of nitrogen compound added depends upon the amount of sulfur contained in the aromatic feed, but preferably the nitrogen compound should be sufficient to make the atom ratio of nitrogen to sulfur in the feedstock at least 0521. It has been found that a sufficient amount of the nitrogen containing compound must be added to provide at least 5 p.p.m. nitrogen based on a sulfur-free feed for any appreciable effect to be detected. The practical upper limit on nitrogen compound addition is 15,000 ppm. although no detrimental effects have been observed even at these nitrogen levels in low sulfur feedstocks. With higher sulfur containing feedstocks, however, high nitrogen content causes rapid fouling of the catalyst. The preferred levels of nitrogen compound addition are thus sufiicient amounts to make the nitrogen content from 5 to 15,000 ppm. of the feedstock and to make the atom ratio of nitrogen to sulfur at least 0.511.

It has been determined that the nitrogen compound should preferably be halogen free. Compounds containing one gram atom of halogen for each gram atom of nitrogen should especially be avoided. The halogen compounds promote isomerization and formation of ammonium halides which have a detrimental effect on the reaction. In particular, the halogen-free organic compounds of nitrogen, such as ammonia, pyridines, pyrroles, nitriles and amines are preferred. The single-preferred nitrogen compound is halogen-free ammonia because it is clean, relatively easy to handle and rapid acting.

The preferred method is to premix ammonia with the hydrogen prior to entry into the hydrogenation reactor. This method enables close control over the nitrogen addition at the hydrogenation facilities. The ammonia concentration should be great enough to provide from 5 to 15,000 p.p.m. nitrogen in the feedstock and should be sufficient to maintain the ratio of nitrogen to sulfur in the reactor at least greater than 0.121 and preferably greater than 0.5 :1. The feedstock, for example benzene, is then hydrogenated in the presence of a platinum catalyst at a temperature from 200 to 400 C. and a pressure of about 500 p.s.i.g. In this method, the liquid hourly space velocity is preferably maintained between 1 and The catalysts of the present invention are the conventional type commonly used in the hydrogenation of unsaturated hydrocarbons which have had substantially all of the sulfur and nitrogen removed before hydrogenation. The catalytically active noble metal is supported on material such as alumina, silica gel, or any other high surface area refractory support, and should comprise from 0.1 percent to 10 percent by weight of the catalyst material, preferably 0.3 percent to 2 percent. Higher percentages of the noble metal can be used, however. Such higher percentages are not practical except possibly for liquid phase hydrogenations. The process of the present invention is best performed as a continuous process in which hydrogen and a vaporized feed containing aromatic hydrocarbons are mixed and then passed through a reactor zone containing the platinum catalyst. When a continuous process is employed, a continuous flow of fresh hydrogen, or at least hydrogen having a low sulfur content (equivalent to less than 2,000 ppm. sulfur based on the hydrocarbon feed) is introduced into the reactor. In batch processes, excessive amounts of hydrogen sulfide tend to be formed unless a very low sulfur content feed is being hydrogenated.

Platinum is the preferred catalyst of the present invention because it apparently possesses unique properties among the normally sulfur-sensitive noble metal hydrogenation catalysts and it functions effectively in the presence of sulfur under the conditions described herein. Other noble meal cataylsts such as palladium and rhodium, while operable, are not as effective in the presence of sulfur and nitrogen as is platinum.

The following examples are operable and preferred embodiments of my invention. Tables I-III summarize the clearly beneficial effect nitrogen compounds have on high temperature hydrogenation of aromatics. These examples are provided to further illustrative my invention and should not be construed as being limitative of the scope of the invention.

EXAMPLES 1-6 TABLE I Product analysis, wt. percent EOP & Temp, C. P.p.m. (S) P.p.m.(N) I MGH l DMGP 3 Toluene Trace 0 97. 7 0. 6 1. 63 200 0 78. 9 2. 2 18. 9 200 200 44. 4 Trace 55. 5 Trace 200 97. 4 Trace 2. 5 500 0 60. 7 5. 0 25. 3 200 200 T 3. 0 Trace 26. 9

l Methylcyelohexano.

Z Ethylcyelopentane+13imethylcyclopentanes.

1 This run carried out at 500 p.s.i.g., 5 LHSV, and a hydrogen-to-tolueno ratio of 9.9.

The data from Table I show that the addition of a nitrogen compound to the reaction reduces the sulfurinduced cyclohexane isomerization to cyclopentanes to trace amounts. The data also show that the amount of isomerization is decreased by adding nitrogencontaining compounds to the reaction even where the initial sulfur content of the feedstock is only trace amounts.

EXAMPLES 7-17 A toluene feedstock was hydrogenated at 500 p.s.i.g., 5 LHSV, and 12,400 c.f. (60 F.) hydrogen per barrel (a molar ratio of hydrogen to toluene of 9.9) using a single 10 ml. charge of a 0.6 percent platinum-on-alumina catalyst (Englehard RD-260). This catalyst is recommended for use where isomerization is not desired. The molar ratio of hydrogen to toluene was 9.9. The results of these runs are set forth in Table II. Nitrogen was added to the feedstock as pyridine except in Example 9 6 where ammonia was preblended with the hydrogen. toluene when hydrogenated contained no cyclopentanes. Even with the large quantity of ammonia used in Example This amount of sulfur and nitrogen at such a low temper- 9, no serious detrimental effect was observed on the hyature, however, severely reduced the rate of hydrogenadrogenation rate or the catalyst life. At the same time, tion of the toluene. Thus, although a reduction in isomerthe addition of large quantities of ammonia, as in Exized products was observed at temperatures as low as ample 9, effected a large reduction in the amount of isom- 5 280 C., when pyridine was added to the sulfur containerization. Generally, from the results of these examples ing aromatic feedstock, the eifectiveness of nitrogen in it can be concluded that hydrogenation of sulfur conreducing isomerization was counterbalanced by the reductaminated feedstocks can be carried out at much higher tion in hydrogenation of the aromatics.

temperatures than previously thought practical if nitrogen Similar runs with lower sulfur feedstocks have shown is added to reduce isomerization of the saturated products. that beneficial results are obtained by adding nitrogen con- TABLE IL-TOLUENE FEED Product, GLC analysis, wt. percent Isomerized Saturated 2 saturated Saturated/ Example Cat. age, hrs. Temp., C. S p.p.m N p.p.m. N [S ratio 1 Toluene product product isomerized 340 3 0. 3 0. 2 1 63 97. 77 0. 60 165 340 3 50 38 0 80 98.85 0. 282 47 340 3 13, 400 10, 000 2 2 97. 6 0. 2 488 51 342 100 0. 3 0.007 1 6 97. 9 0.5 196 59 340 100 50 1. 14 1 47 98. 21 0. 32 306 34 341 500 0 3 0. 001 24 74. 96 0. 64 117 62 340 500 0.23 13 6 86.11 0.32 269 10 370 3 0 3 0.2 4 13 95.11 0.76 125 70 371 500 0 3 0.001 8 72 88.22 3.06 27 65 370 500 50 0.23 8 32 90 92 0.81 112 76 370 500 250 1. 14 19 95 79 71 0. 228 1 Sulfur present as carbon disultlde, nitrogen as pyridine except Example 9; N S ratio is on atomic basis. 2 Saturated product is methylcyclohexane; isomerized saturated product is a mixture of ethylcyclopentane and dimethylcyclopentanes. 3 Average of three runs with ages of 12, 43 and 96 hours respectively. 4 Nitrogen present as ammonia blended with hydrogen gas (0.73 mole percent N H3 in Ha).

EXAMPLES 18-21 taining compounds during hydrogenation at temperatures 0 A benzene feed hydrogenated at about 370 C., 500 30 as am 333 g tions and var.ations of th S p.s.i.g., 5 LHSV, and 12,400 c.f. (60 F.) hydrogen y l e 1 n a hereinbefore set forth, may be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the per barrel using the same 10 ml. charge of catalyst as used in Examples 7-17. The molar ratio of hydrogen to benzene was 8.3. Again with benzene, when nitrogen 5 d d 1 in the form of pyridine was added, a definite beneficial 3 i 9 aims effect is shown in Table III on the isomerization of the c saturated aromatics. This effect may best be observed A Process for Producmg cyclohexanes compnsmg from a comparison of the ratio of saturated to isomerized the steps Products in Table In This value shows that for even contacting an aromatic feedstock containing up to 2500 small amounts of sulfur the addition of nitrogen com- 40 sulfurowlth hydrogen at a temperature of from pounds increases the ratio of saturated to isomerized to i 3 3 Pressure from} 250 to 1500 products, about 6 times. As Examples 20 and 21 show, Wlth a Platlmlm catalyst and Wlth 5 to 15,000 when the ratio of nitrogen to sulfur was 0.5 :1, the ratio P-P- Of an added halogen-free nitrogen Containing of saturates to isomerized products was increased by Compound selected from the group Consisting of greater than 4 times. 45 amine and ammonia; and

Examples 18-21 and Table III further illustrate that maintaining the ratio of atoms of nitrogen to atoms of the nitrogen additions have substantially no detrimental sulfur at greater than 0.1:1 to produce substantially influence on the catalyst activity or life and that much isomer-free saturated products. higher temperatures than heretofore thought desirable 5 2. A process as defined in claim 1 wherein said nitrogen can be tolerated in the hydrogenation reactor without containing compound is ammonia. excessive isomerization or reduction in hydrogenation 3. A process as defined in claim 2 wherein said arorate. matic feedstock comprises benzene.

TABLE I'lL-BENZENE FEED Product, GLC analysis, wt. percent Isomerizcd Z Saturated Z saturated Saturated] Example Cat. age, hrs. Temp, "C. S p.p.m. N p.p.mJ N /S ratio 1 Toluene product product isomerized 121 370 9 0. 6 0. 15 3. 31 95. 84 0. 85 113 108 370 9 250 64 3. 70 96. 15 0. 15 640 83 370 500 0. 6 0. 003 9. 32 89. 87 0.81 111 81 371 500 250 1. 14 20. 83 73. 01 0. 16 456 1 Sulfur present as carbon disulfide, nitrogen as pyridine; N/S ratio is on atomic basis. 2 Saturated product is methylcyclohexane; isomerized saturated product is a mixture of ethylcyclopentane and dimethylcyclopentanes.

EXAMPLE? 22 23 4. A process as defined in claim 1 wherein said aro- A toluene feedstock Contalnlng Varylng amounts of matic feedstock is contacted with said added nitrogen sulfur in the form of carbon disulfide was hydrogenated compound by premixing said nitrogen compound with said in the hydrogenation reactor at 800 p.s.i.g., 20 LHSV, feedstock prior to contacting said feedstock with hydrogen. and a 5:1 molar ratio of hydrogen to oil, using a low 5. A process as defined in claim 1 wherein said added chloride catalyst (RD260). At 280 C. with 200 p.p.m. nitrogen compound is contacted with said feed-stock by of sulfur and no nitrogen containing compounds, the premixing said added nitrogen compound with the hydroproducts contained 0.1 percent by weight isomerized cygen prior to contacting said hydrogen with said feedstock. clopentanes and 29.2 percent by weight cyclohexanes. At 6. A process as defined in claim 1 wherein said aroabout the same temperature with 200 p.p.m. of nitrogen matic feedstock is benzene containing up to 500 p.p.m. added in the form of pyridine, the same sulfur containing 7 sulfur.

7. A process for forming cyclohexanes from sulfur containing aromatic feedstocks comprising:

providing a platinum catalyzed hydrogenation reaction zone having a temperature from 280 to 400 C. and a pressure sufficiently high to promote hydrogenation of said aromatic feedstock; contacting said feedstock with hydrogen and an added halogen-free nitrogen containing compound selected from the group consisting of an amine and ammonia in said reaction zone for a sufficient length of time to hydrogenate said feedstock; maintaining the nitrogen content in said reaction zone at from 5 to 15,000 ppm. of said feedstock and suflicient to provide a nitrogen-to-sulfur atomic ratio of greater than 0.121 to suppress isomerization of the hydrogenated product; and recovering said hydrogenated product. 8. A process as defined in claim 7 wherein said feedstock comprises benzene containing up to 500 ppm. sulfur and said product comprises cyclohexane.

9. A process as defined in claim 7 wherein said nitrogen content in said reaction zone is maintained sufficiently high to provide a nitrogen-to-sulfur atomic ratio of greater than 0.5:1.

References Cited UNITED STATES PATENTS 2,914,470 11/ 1959 Johnson 208264 3,3 94,077 7/ 1968 Kovach 208-2 17 2,898,387 8/1959 Teter 260667 3,197,398 7/1965 Young 260-667 3,317,419 5/1967 Fortman 208143 3,269,939 8/1966 Marechal 208-143 3,254,134 5/1966 Smith 260667 DELBERT E. GANTZ, Primary Examiner.

V. OKEEFE, Assistant Examiner.

US. Cl. X.R. 208264 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,446,863 May 27, 1969 Frederick W. Steffgen It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 30, "illustrative" should read illustrate line 36, "feedback" should read feedstock Columns 5 and 6, TABLE II, the next to last column heading entitled "Isomerized saturated product" should have a superscript "2" relating to the footnote "2"; the last figure in this column, "0.55" (relating to Example 17] should read 0.35 in the last column entitled "Saturated/isomerized", the first figure in this column (referring to Example 7) "165" should read 163 same columns, TABLE III, footnote 2, "isomeriZed" should read isomerized Signed and sealed this 3rd day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

